1. Field of the Invention
This invention relates to methods and systems for location finding. More specifically this invention relates to location finding methods and systems, which make use of “Time Difference of Arrival.”
2. Description of Related Art
A variety of methods and techniques have been proposed for geographic location determination. The most widely used system is the Global Positioning System (GPS). The GPS system is a satellite-based pseudo-ranging location system that is used widely in military and commercial applications. Typical GPS accuracy is less than 10 meters for military applications and 100 meters for commercial applications. Differential GPS (DGPS) has been introduced to improve the location accuracy for commercial applications. GPS provides accurate position information for users in open and unobstructed areas. It requires that at least three satellites be within range of the GPS receiver to provide 2-D position data and four or more satellites to provide 3-D data. Satellite blockage caused by man made or natural structures within urban and/or shadowed environments can render GPS inoperable. Generally such GPS systems do not provide the broad outdoor coverage in an urban and/or shadowed environment. Also, such GPS systems tend to be relatively costly.
Other common position location methods include Direction Finding (DF), hyperbolic methods and Time of Arrival (TDOA). DF techniques generally require steerable antennas or phased-array antenna systems. Although DF techniques can offer substantial spatial resolution and have the ability to locate a number of signals, typically DF systems are highly complex. The complexity translates to high cost, low reliability and often imposes size and power restrictions on the use of the system. While TOA is a simple triangulation method, it requires both the base stations and all of the mobile devices to have highly accurate clocks. This requirement for highly accurate clocks imposes a significant cost impact on the use of the TOA method.
Although the following may not qualify as “prior art,” the reader is referred to the following U.S. patent documents for general background material. Each of these referenced patents is hereby incorporated by reference in its entirety for the material contained therein. U.S. Pat. Nos. 3,996,590; 4,060,809; 4,252,999; 4,783,744; 5,689,269; 5,752,218; 5,781,150; 5,952,959; 5,963,130; 6,041,222; 6,061,337; 6,104,978; 6,184,801; 6,198,390; 6,204,813; 6,236,365; 6,252,544; 6,249,252; 6,295,023; 6,324,213; 6,353,406; 6,388,611; 6,392,692; 6,424,826; 6,429,811; 6,429,812; 6,452,544; 6,459,704; 6,497,656; 6,509,829; and 6,560,462.